This appliance doesn't have a thermostat as such. It has an electronic control board but if the freezer is ok but the fridge isn't then its not a fridge thermostat fault.
What you need to do is empty the freezer and remove the back panel in there as the fridge gets its cooling from the freezer. Check to see if the evaporator in there is blocked with ice.
If so then you need to melt it with a hairdyer and then see how it goes from there. As the ice build up on the evaporator can stop the air flow up into the fridge section.
If this is blocked with ice then you need to test the defrost heater which is wrapped around the evaporator. You will see the wiring blocks for it in there. Test the resistance of them and see if faulty.
If the heater is fine then you need to test the 2 NTC (temperature sensor) you will see 1 clipped to the back panel and the other in the evaporator. The wiring blocks needs are yellow and green. Test these as well and see what resistance you get. They should be between 4 and 8 Kohms roughly. If any are faulty then they will need to be replaced.
Check them out and see what you find an let me know as then I can get part numbers for you.